Do arid-zone ants undergo boom-bust population fluctuations in response to the El Niño Southern Oscillation?

A project undertaken at the Department of Zoology, La Trobe University, and supervised by Heloise Gibb

Arid ecosystems in Australia undergo repeated cycles of boom and bust in response to quasiperiodic swings in climate patterns resulting from the El Niño Southern Oscillation (ENSO). Dramatic fluctuations in populations of many animal species result from rainfall-driven changes in food availability. These boom-bust cycles may make species particularly vulnerable to anthropogenic extinction drivers, including altered fire regimes, predation by feral animals and habitat disturbance through grazing. However, long-term data sets are required to understand these cycles, and for invertebrates, such data sets are rare.

This was the first long-term study testing responses of insect assemblages to ENSO-driven rainfall fluctuations in the Australian arid zone. We also examined the role of spatial heterogeneity in moisture levels in buffering climatic effects by comparing productive 'refuges' with unproductive 'non-refuge' sites. We focussed on changes in the abundance of ant species and the rate at which they perform ecosystem functions, including seed dispersal, predation and scavenging. Early results suggest that ant populations follow similar boom-bust dynamics to other taxa, although fluctuations may be buffered by the colonial nature of this taxon. Fluctuations in low and high productivity components of the landscape are similar, with some ant taxa more abundant at low productivity sites. The abundances of competitively dominant ants, such as Iridomyrmex, appear to undergo more pronounced fluctuations than seed harvesters, with predators intermediate. These improved insights into the long-term responses of insects to boom-bust cycles allow us to better understand associated changes in ecosystem function and the bottom-up forces driving fluctuations in taxa at higher trophic levels.

Figure 1. Arid zone ants of the genus Calomyrmex (photo by H Gibb)

Figure 2. Simpson Desert dune (photo by Aaron Greenville)

Figure 3. Contrasting low and high productivity areas from above (Image from GoogleEarth)